1. Field of the Invention
The present invention relates to a common mode filter, and more particularly relates to a common mode filter configured by using a drum core.
2. Description of Related Art
There is a known common mode filter that is provided on each of two signal lines constituting a transmission path using a differential transmission method, and that is configured by two inductances magnetically coupled with each other. By inserting the common mode filter into the transmission path using a differential transmission method, it is possible to selectively remove only a common-mode noise current.
It is known that a toroidal core or a drum core is used as a specific structure of the common mode filter. In the case of using the toroidal core, a leakage flux can be suppressed as compared to the case of using the drum core, and therefore high noise-removal performance can be obtained. On the other hand, because automatic coil winding is difficult for the toroidal core, it inevitably requires manual coil winding, thereby increasing variations in characteristics of the common mode filter. In contrast to this, in the case of using the drum core, it is difficult to obtain as high noise-removal performance as that of the toroidal core. On the other hand, an automatic coil winding method can be used, thereby lessening variations in characteristics of the common mode filter. Further, because the automatic coil winding method can be utilized, a drum-core type common mode filter is suitable for mass production.
Japanese Patent Nos. 4789076 and 3973028 disclose an example of a common mode filter configured by using a drum core. In the example of Japanese Patent No. 4789076, two wires each of which constitutes an inductance are wound with a double-layer structure. In contrast, in the example of Japanese Patent No. 3973028, two wires each of which constitutes an inductance are wound together as a pair of wires. Generally, the former winding method is referred to as “layer winding”, and the latter winding method is referred to as “bifilar winding”. Furthermore, Japanese Patent No. 4737268 discloses an example of an automatic coil winder that is used to wind a wire around a drum core.
In recent years, Ethernet has been widely adopted as an in-vehicle LAN. A common mode filter used in in-vehicle Ethernet is required to have more stable characteristics and higher noise-reduction performance than ever before. In this respect, a drum-core type common mode filter has a feature of being able to lessen variations in its characteristics, as described above. Therefore, when noise-reduction performance of the drum-core type common mode filter can be improved, it is possible to obtain the optimized common mode filter for in-vehicle Ethernet.
What is specifically required as high noise-reduction performance is reduction in mode conversion characteristics (Scd) which indicate the rate of a differential signal component, input to a common mode filter, to be converted into a common mode noise and to be output. As a result of extensive studies by the present inventors in order to satisfy the requirement, it has been found that a capacitance caused between different turns (hereinafter, “capacitance between different turns”) is closely associated with the reduction in the mode conversion characteristics in a common mode filter configured by using a drum core. The mode conversion characteristics are reduced by reducing the capacitance between different turns.
As disclosed in Japanese Patent No. 3973028 for example, the capacitance between different turns can be reduced by providing a given space (a “b” portion in FIG. 2 in Japanese Patent No. 3973028) between pairs of wires wound by bifilar winding. Such a winding method is employed in Japanese Patent No. 3973028 for the purpose of increasing a cutoff frequency, and there is no description of the mode conversion characteristics in Japanese Patent No. 3973028. On the other hand, in the winding method as described in Japanese Patent No. 3973028, each of spaces of equal width is provided between any two adjacent turns, and therefore the number of windings is decreased. The actual number of windings in the common mode filter in Japanese Patent No. 3973028 is only four turns, and an inductance (300 μH), required for a common mode filter utilized in in-vehicle Ethernet, cannot be obtained from the four turns.